support nulls in belongsTo

[gpibarra]

This is an improvement on PR #975.

The initial motivation is to support the search for belongsTo=null.

In these cases, it is necessary that the database, the column allows nulls, for example $table->foreignId('user_id')->nullable()->constrained(table: 'users', indexName: 'posts_user_id');.

In cases where it is detected that a nullable value is sent (null, 'null', 0, '0', ''), then whereNull($column) is added to the query.

In this PR the code was simplified, instead of using whereBelongsTo($collection, $relation), whereIn($relation->getQualifiedForeignKeyName(), $values) was used; where $values is no longer a collection but an array (with the ids).

In cases where a nullable value and an id are sent at the same time, for example [null, 1, 3], both wheres are executed using the OR operator:
$query->where(fn () => $q->orWhereNull($relationColumn)->orWhereIn($relationColumn, $values)

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As I said in #975

Unfortunately my English is not good and I couldn't write the documentation correctly.
If you think the PR is correct, could you help me with the documentation?

The documentation is pending.
https://github.com/spatie/laravel-query-builder/blob/main/docs/features/filtering.md#belongsto-filters